Glucose-dependent insulinotropic polypeptide (GIP) is an incretin secreted by secretory cells of the gastrointestinal epithelium (K cells) in response to the intake of lipids, carbohydrates, and amino acids. GIP promotes insulin secretion from pancreatic β cells in a glucose-dependent manner, thereby contributing to the regulation of blood glucose levels. Recently, mice with artificially elevated blood GIP concentrations are reported to have exhibited suppressed fat burning when given a high-fat diet. In addition, it has been revealed that GIP receptor-deficient mice exhibit suppressed visceral and subcutaneous fat accumulation induced by a high-fat diet. From these findings, the postprandial GIP regulation is considered to be effective for the prevention and amelioration of obesity. Further, GIP is known to have suppressing actions on gastric acid secretion and gastric motility; therefore, suppression of GIP level elevation is assumed to be effective for promoting digestion after meal and ameliorating heavy feeling in stomach. In light of the foregoing, there is a demand for the development of a substance which suppresses GIP level elevation. To accomplish this, there is a demand for the development of a rapid and highly sensitive method to evaluate the capacity of a substance to suppress GIP level elevation.
As a conventional method for evaluating or selecting an agent for suppressing GIP level elevation, a method using the expression of the CPT1 gene and CPT1 protein, or the activity of the CPT1 protein in cells as the index (Patent Literature 1), a method using the expression of the FAT/CD36 gene or FAT/CD36 protein as the index (Patent Literature 2), and the like are known.
Fatty acid-binding proteins (FABPs) are the members of the intracellular fatty acid-binding protein family, and are 14 to 15 kDa proteins having high binding affinity to fatty acids (Non Patent Literatures 1 and 2). Fatty acids exhibit various functions in cells such as the energy source and signaling molecules in metabolic regulation. FABPs enable transport of insoluble fatty acids to various organelles by binding to them, thereby playing an important role in the functional expression of fatty acids.
FABP4 and FABP5, which are isoforms of FABPs, are highly homologous in their amino acid sequences and steric structures, and are co-expressed in adipocytes and macrophages (Non Patent Literatures 2 to 4). Functional analyses of FABP4 and FABP5 have been conducted using knockout mice (Non Patent Literatures 2 to 15). FABP4 accounts for 1 to 3% of cytosolic proteins in adipocytes, and is widely used as a differentiation marker of adipocytes (Non Patent Literature 5). It is reported that FABP4 not only functions as a molecular chaperone in adipocytes, but also is involved in lipid-mediated signal transduction, responses in organelles, and further, inflammatory responses in macrophages (Non Patent Literatures 2, 6, and 9). FABP5 is suggested to be associated with the formation of psoriatic lesions in keratinocytes (Non Patent Literature 7) and is reported to be associated with the regulation of a cytokine (IL-12p70), which is a key molecule of the innate immune response in the spleen (Non Patent Literature 8).
In the ob/ob mouse, which is the type-II diabetes model, in which FABP4 was knocked out, a decrease in insulin sensitivity was suppressed (Non Patent Literature 11). Compared to the wild type, a decrease in insulin sensitivity was suppressed while there was no impact on the body weight gain and fatty liver in FABP4-knockout mice on a high-fat diet (Non Patent Literatures 4 and 10). Since the expression of FABP5 is increased in adipocytes of FABP4-knockout mice, FABP5 is considered to work in a compensatory manner (Non Patent Literatures 16 and 17). FABP5-knockout mice also show a similar tendency to FABP4-knockout mice (Non Patent Literature 12). In light of the results obtained with single-knockout mice, FABP4/5 double knockout was analyzed. As a result, compared to the wild type, the induction of diet-induced obesity, insulin resistance, type-II diabetes, and fatty liver is reported to have been suppressed in double-knockout mice on a high-fat diet (Non Patent Literatures 13, 14, and 15).
Amelioration in the pathological condition was observed in the mouse model of type-II diabetes or arteriosclerosis receiving the oral administration of BMS309403, which is an FABP inhibitor (Non Patent Literature 18). Administration of an FABP4/5 inhibitor to diet-induced obese mice resulted in amelioration in dyslipidemia, which was observed as, for example, a decrease in the blood triglyceride and free fatty acid levels (Non Patent Literature 19).
However, given that FABP2 is the type of FABP predominantly expressed in the intestine, it has been conventionally believed that FABP2 plays a role in lipid metabolism in the intestine.